Moderation of Negative Oxygen Effects by Small Yttrium Addition to Low Activation Vanadium Alloys

In order to improve irradiation embrittlement of vanadium alloys for fusion reactors, yttrium (Y) has been added reducing the interstitial oxygen impurity. However Y addition can also degrade high-temperature strength, because Y could scavenge oxygen in solid solution, which is a strong hardening agent in vanadium alloys. In this study, the effect of Y addition and oxygen level on the mechanical properties was investigated from the view points of both the high-temperature strength and low temperature ductility. Y addition was suggested to moderate the hardening and embrittlement induced by oxygen impurity sustaining the high-temperature strength within an acceptable level

Effect of yttrium on dynamic strain aging of vanadium alloys

In order to improve the performance of vanadium alloys for fusion reactors, yttrium (Y) was added to reduce the interstitial O in the matrix by enhanced precipitation with Y. Effect of Y on interstitial C, N and O, however, remains to be investigated since they affect mechanical and fracture properties for vanadium alloys by pinning dislocations, such as dynamic strain aging (DSA). In this study, tensile tests were carried out on annealed V–4Cr–4Ti and V–4Cr–4Ti–Y alloys from 473 to 1073 K at strain rates ranging from 6.67 × 10−5 to 6.67 × 10−1 s−1 to investigate the performance of DSA. In the case of high-purity alloys, DSA regime was narrowed due to Y addition and the reduction in O content. In the case of O doped V–4Cr–4Ti alloys, DSA regime was also narrowed. This may be because the enhanced Ti–O precipitation reduced the O level in the matrix. Also, coarse precipitates (<500 nm) were observed in O doped V–4Cr–4Ti–Y alloy. Y might enhance the coarsening of Ti-precipitates. From activation energies for DSA, the diffusion of C and O is considered to induce the observed DSA. Y does not influence the diffusion of C and O, and might enhance the nucleation to form coarse precipitates

Evaluation of irradiation hardening of ion-irradiated V–4Cr–4Ti and V–4Cr–4Ti–0.15Y alloys by nanoindentation techniques

Irradiation hardening behavior of V–4Cr–4Ti and V–4Cr–4Ti–0.15Y alloys after Cu-ion beam irradiation were investigated with a combination between nanoindentation techniques and finite element method (FEM) analysis. The ion-irradiation experiments were conducted at 473 K with 2.4 MeV Cu2+ ions up to 7.6 dpa. For the unirradiated materials, the increase in nanoindentation hardness with decreasing indentation depth, so-called indentation size effect (ISE), was clearly observed. After irradiation, irradiation hardening in the measured depth was identified. Hardening behavior of bulk-equivalent hardness for V–4Cr–4Ti–0.15Y alloy was similar to that for V–4Cr–4Ti alloy. Y addition has little effect on irradiation hardening at 473 K. Adding the concept of geometrically necessary dislocations (GNDs) to constitutive equation of V–4Cr–4Ti alloy, the ISE was simulated. A constant value of α = 0.5 was derived as an optimal value to simulate nanoindentation test for ion-irradiated V–4Cr–4Ti alloy. Adding the term of irradiation hardening Δσirrad. to constitutive equation with α = 0.5, FEM analyses for irradiated surface of V–4Cr–4Ti alloy were carried out. The analytic data of FEM analyses based on neutron-irradiation hardening equivalent to 3.0 dpa agreed with the experimental data to 0.76 dpa. The comparison indicates that irradiation hardening by heavy ion-irradiation is larger than that by neutron-irradiation at the same displacement damage level. Possible mechanisms for extra hardening by heavy ion-irradiation are the processes that the injected Cu ions could effectively produce irradiation defects such as interstitials compared with neutrons, and that higher damage rate of ion-irradiation enhanced nucleation of irradiation defects and hence increased the number density of the defects compared with neutron-irradiation

Effect of neutron irradiation on Nb3Sn wire

A Nb3Sn wire which was manufactured for the ITER toroidal field coil conductor by a bronze route process was prepared for this study to investigate the effect of neutron irradiation on the critical current in a high magnetic field. The critical current of the virgin wire was measured in liquid helium with a 28 T hybrid superconducting magnet at the High Field Laboratory for Superconducting Materials in Tohoku University. It was also measured in vacuum with a heat conduction type variable temperature insert (VTI) at the International Research Center for Nuclear Materials Science at Tohoku University. The wire was irradiated at below 100 °C by fission neutrons at up to 4.9 × 1022 neutrons m−2 (>0.1 MeV) at BR2 in Belgium, and the critical current after the irradiation was evaluated with a VTI in the range of 8–15.5 T. The difference of the critical current measured with two facilities was discussed, focussing on Joule heating of the sample holder which was made of pure copper, and the neutron irradiation effect on the critical current was investigated in the range of up to 15.5 T. The results show that the critical current measured in vacuum becomes lower than that in liquid helium because of the temperature rise of the sample holder where the sample was soldered, the critical current was increased by the neutron irradiation, and the current ratio (IC/IC0) was almost constant of 1.75 in the range of 8–15.5 T at around 4 K

Effect on impact properties of adding tantalum to V-4Cr-4Ti ternary vanadium alloy

Four V-Ta-4Cr-4Ti quaternary alloys containing different quantities of Ta were investigated to determine the effect of Ta content on the Charpy impact properties. Five button-shaped ingots of the V-4Cr-4Ti ternary alloy and V-xTa-4Cr-4Ti quaternary alloys (x = 3, 9, 15, and 22 wt.%) were fabricated on a laboratory scale by using non-consumable arc-melting in an argon atmosphere. Charpy impact tests were conducted at temperatures ranging from 77 K to 293 K using an instrumented impact tester. Both the upper shelf energy and the ductile–brittle transition temperature increased with increasing Ta content. The addition of 3 wt.% Ta resulted in solid solution strengthening without any degradation of the Charpy impact properties. Thus, the addition of 3 wt.% Ta (V-3Ta-4Cr-4Ti) is an appropriate amount to use in blanket structural materials for nuclear fusion reactors. The spectra of TEM-EDS for V-3Ta-4Cr-4Ti quaternary alloy indicate that there is no significant enrichment of Ta in the matrix as compared with that in the precipitate. However, thermal aging may result in the formation of the Laves phase, causing the degradation of Charpy impact properties. The characterization of precipitates, thermal aging, and creep tests of the V-3Ta-4Cr-4Ti quaternary alloy need to be investigated to determine the optimum Ta content

Effect of tantalum addition on the tensile properties of V-Ta-4Cr-4Ti quaternary alloys

Various V-Ta-4Cr-4Ti quaternary alloys containing different amounts of Ta were investigated to determine the effect of Ta concentration on their tensile properties. Six button-shaped ingots of V-xTa-4Cr-4Ti alloys (x = 4, 8, 15, 22, 24, and 35 wt.%) were fabricated on a laboratory scale by using non-consumable arc-melting in argon atmosphere. The sheet specimens for all the V-Ta-4Cr-4Ti alloys examined herein were obtained by cold-rolling; hot-rolling or intermediate annealing was not required. Although the examined alloys were fabricated on a laboratory scale, the high-Ta alloys, unlike the high-Cr alloys, did not experience crack formation in the cold rolling process. It is possible that V-4Cr-4Ti alloys adopt large amount of Ta and its strengthening effect without negative impact on rolling fabricability. Tensile strength tended to increase with increasing Ta content at both room temperature and high temperatures of 973 K and 1073 K, which can be attributed to the solid solution strengthening by Ta. Thus, addition of Ta to V-4Cr-4Ti alloys can improve their strength without decreasing their rolling fabricabilities. However, an alloy containing excess Ta (V-35Ta-4Cr-4Ti) showed a loss of ductility and a brittle fracture mode due to the formation of a large amount of friable Laves phase. Therefore, it is necessary to limit the amount of Ta to prevent the formation of the Laves phase